The removal of nitrogen oxides (hereinafter NOx) from the exhaust gases of internal combustion engines is required for cleaner operating vehicles. Improvements in fuel efficiency are achieved by operating at conditions with an excess of air than required for stoichiometric combustion (i.e., lean burn or rich conditions). Such “lean burn” conditions are commonly achieved in diesel engines and four cycle engines. However when lean-burn conditions are employed, common pollution reduction devices (e.g., three-way catalysts) are inefficient in the reduction of nitrogen oxides.
One approach to reduce nitrogen oxide pollutants in exhaust gases of engines operating under lean-burn conditions has been to incorporate a non-thermal plasma reactors in the exhaust lines along in addition to the standard three-way catalyst. Such reactors treat the exhaust gases using a non-thermal plasma field. The non-thermal plasma field is a high local electric field. The plasma converts NO to NO2, the NO2 must then be subsequently reduced by a selective catalyst. For example, a non-thermal plasma reactor is described in U.S. Pat. No. 6,139,694, the contents of which are incorporated by reference herein.
Non-thermal plasma reactors include a non-thermal plasma-generating substrate (“substrate”) disposed within a housing. The substrate includes a pair of dielectric plates spaced from one another to form an exhaust gas flow channel. Preferably, the dielectric plates are non-conductive materials such as quartz, glass, alumina, mullite, and oxide free ceramics (e.g., silicon nitrite, boron nitrite, aluminum nitrite). A voltage supply is connected to a pair of electrodes on each dielectric plate for providing a voltage between the dielectric plates in order to generate the plasma field in the flow channel between the plates. The exhaust gas flows through the flow channel, exposing the gas to the plasma field. The plasma field converts NOx into either individual elemental diatoms O2 and N2 and/or nitrogen dioxide NO2.
The flow channels in the reactor are preferably long, narrow rectangular gas channels. However, such long, narrow substrates are prone to crushing due the forces necessary to restrain the substrate in the housing. The plates of the substrate are also prone to arcing of voltage from the plates to the housing. Moreover, the substrate is subject to heating and cooling cycles, which places an additional strain on the substrate. These factors and others create obstacles with respect to retaining the substrate in the reactor.